1. Field
The present disclosure relates generally to analog-to-digital converters and, more specifically, to analog-to-digital converters that receive low currents from voltage signals at their inputs.
2. Discussion of the Related Art
Many electronic devices process information in the form of binary digits. The devices may be as complex as a smart mobile telephone or as simple as a controller for a household appliance. The binary digits often represent the value of an analog signal that the device receives as an input. In those devices that represent an analog input signal in digital form, an analog-to-digital converter may be used to translate the analog signal into a sequence of binary digits that may be processed by digital circuits. A common type of analog-to-digital converter uses a delta-sigma modulator to receive the analog signal.
A controller for an off-line power converter is one example of an electronic device that may use a delta-sigma modulator to receive an analog signal. In a controller for a power converter, the analog signal may represent the measurement of an input voltage that an analog-to-digital converter translates to a sequence of binary digits. The input voltage of an off-line power converter is typically a sinusoidal waveform with amplitude greater than 150 volts. In many parts of the world, the common household voltage that would be the input to an off-line power converter has a peak value greater than 300 volts, and may reach nearly 375 volts under transient conditions. Controllers often measure the input voltage for several purposes, such as, regulating an output voltage or shaping the waveform of an input current to follow the waveform of the input voltage.
Circuits that measure a high input voltage typically do so by using a potential divider, such as a resistive divider, across the input to provide a known fraction of the input voltage as a signal voltage that is low enough for the measurement circuit to handle. In order to reduce power consumption, the components of the divider are selected to minimize the amount of current the circuit draws from the input voltage. However, the current needs to be large enough to guarantee a reliable measurement in the presence of noise.
A circuit, such as a common delta-sigma modulator, that measures the reduced-signal voltage provided by the potential divider, typically takes current from the voltage that it measures. Although the delta-sigma modulator takes current from the reduced-signal voltage provided by the potential divider, that current is obtained from the high voltage source of the input voltage and is passed through the potential divider. Since the power taken from the source of the input voltage is proportional to the product of the voltage and the current, and because the peak value of the input may be hundreds of volts, even the smallest current sufficient for reliable measurement can still result in a significant loss of power, especially when the power converter has a light load or no load.
Thus, since circuits with high input impedances take less current from a given voltage than a circuit with low input impedance, a delta-sigma modulator with high input impedance is desired to reduce the power consumption of circuits that measure relatively high voltages.